This invention generally pertains to an injection molding apparatus. More specifically, the present invention relates to an injection molding nozzle for such an injection molding apparatus.
The invention is particularly applicable to a nozzle for the injection of a viscous fluid, such as a molten plastic, and a non-viscous fluid, such as a gas, into an injection mold during a process such as gas augmented injection molding of plastic materials. However, it will be appreciated by those skilled in the art that the invention has broader applications and may also be adapted for use in many other injection molding environments where both a relatively viscous fluid, such as plastic or wax, and a relatively non-viscous fluid, such as gas, are injected into a mold cavity.
Injection molding processes have been widely used not only for the production of molded articles made of various thermoplastic resins but also for the production of lost wax masters used in the investment casting process. The solid injection molding process generally uses a thermoplastic material.
Solid injection molding employs the steps of injecting a plasticized (melted) thermoplastic material under high pressure into a finite mold space and then allowing the material to cool sufficiently so that it rehardens to the extent that it can retain its shape after removal from the mold. Thermoplastic materials, however, generally shrink during rehardening and, unfortunately, this shrinkage is exaggerated in heavier wall sections, bosses, ribs, gussets, etc. This usually results in sink marks and warpage in the molded product.
Packing the mold with more material by pressing the plastic material at a higher pressure into the mold is a common technique used to minimize such excessive shrinkage. However, packing builds internal stresses in the part and often cannot remove sink marks that are located away from the injection molding sprue or gate. Additionally, packing requires high clamp pressures between the parts of the mold body in order to prevent flashing of the plastic material.
Certain proposals have recently been made to fill the mold cavity with a plasticized thermoplastic material to a specific volume less than 100% of the mold space and utilize an inert gas injected under pressure into the partially plasticized material as it is cooling and rehardening. The gas enters the part and moves along paths of least resistance therein. Such paths are normally in areas where the thermoplastic body is thicker and has slower cooling sections, such as ribs, flow channels, chamfers, etc. In this way, with a suitably designed part, a continuous network of hollowed out sections can be provided. The material displaced by the gas from the middle of the sections moves out to fill the remainder of the mold space. This network of gas channels provides a uniform pressure distribution system throughout the mold space during part rehardening and cool down, thus minimizing internal stresses.
The outer surfaces of thicker sections do not sink because gas has cored them out from the inside and gas pressure holds the plastic material up against the mold surfaces during rehardening. Sink in these sections takes place internally rather than on the exterior surfaces of the part. Since the pressure used for final filling of the part is confined to an area defined by the system of gas flow channels, the resultant force against the sections of the mold is relatively modest so that lower clamping forces on the mold are adequate.
Various types of shut off nozzles for injection molding fluid plastics or wax are known. However, most of these are not adapted for the injection of both a viscous fluid, such as a thermoplastic material and non-viscous fluid, such as a gas, into a mold cavity. Most of those nozzles which are so adapted, do not discharge the gas back through the nozzle when discharge of the gas is required. Instead, in these nozzle systems, the nozzle is spaced away from the mold body in order to vent the gas pressure within the mold cavity. Even those nozzles which are adapted to vent the gas back through the nozzle are unsatisfactory because the molten plastic remaining in the nozzle or in the mold space is frequently vented back along with the gas. This can be deleterious to the gas lines in the nozzle and to the gas piping and valves downstream from the nozzle. Also, if such plastic solidifies in the gas lines in the nozzle, the nozzle becomes unusable until it is cleaned out, which is a time consuming, difficult, and expensive process.
Accordingly, it has been considered desirable to develop a new and improved injection molding nozzle which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.